The present invention relates to a data sampling circuit which deals with a radio signal, and to a portable radio terminal device such as a pager which incorporates the data sampling circuit. In particular, the present invention relates to a data sampling circuit for a decoder of a pager which receives a radio signal and detects a user specific number contained therein to initiate a given processing procedure.
A radio paging operation takes place with respect to a small size radio receiver such as a pager carried by a person who is off his or her office or desk to let him or her know the occurrence of a need of communication by a calling tone, a vibration or a displayed message.
A radio paging system in the pager includes POCSAG (Post Office Code Standardization Advisory Group) coding scheme. A code word used in the POCSAG coding scheme has four types including a synchronization (sync) code word, an address code word, a message code word and an idle code word, each consisting of 32 bit data.
In general, a conventional data sampling circuit includes a 32 bit shift register, namely, a shift register having 32 stages, and an output from each stage is connected to a preamble detector and a sync code word detector, respectively. When a thirty second bit data is sampled by the shift register in accordance with a receiving timing of the address code word or the message code word which consists of 32 bit data, the 32 bit data contained in the shift register is transferred in a serial manner to the error correction unit in synchronization with a system clock before the sampling of a next 32 bit data is started.
However, with the above-mentioned conventional data sampling circuit as used in a digital portable radio terminal device, if the system clock has 32 clocks or less in an interval corresponding to a data length of a single bit of the radio signal, the sampling of a next 32 bit data will begin before the reception of the 32 bit data is completed. With the result that, there is a problem of a misalignment occurring in the data sampling in unit of 32 bits.